This invention relates to variable control arrangements for valves such as internal combustion engine lift valves.
In internal combustion engines such as those used in motor vehicles, there is often a need to vary the valve open times and the valve lift strokes of the intake and exhaust valves, especially the intake valves, as a function of one or more operating parameters of the engine, such as the engine speed and/or load. German Offenlegungsschrift No. 38 15 668 discloses a variable valve lift arrangement having a hydraulically adjustable coupling assembly consisting of a cup plunger and an inner plunger forming a hydraulic working space and disposed between a valve stem and a valve-actuating cam. In this coupling assembly, a compression spring moves the cup plunger and the inner plunger apart by a maximum distance, providing the maximum lengthwise extent of the hydraulic working space, during the base circle phase of the cam, when the lift valve is closed. At that time, the hydraulic working space is in flow communication with an accumulator for a hydraulic fluid pressure medium. As soon as the cam produces a longitudinal displacement of the cup plunger, the pressure in the working space is increased. Depending on the position of a solenoid valve which provides a flow passage to the accumulator when open, a pressure reduction or limitation occurs in the hydraulic working chamber so that its lengthwise extent, and hence the lengthwise distance between the cup plunger and the inner plunger, is decreased. If the solenoid valve is not actuated, this conventional valve arrangement operates with the maximum stroke of the lift valve and maximum valve open time. By actuating the solenoid valve, the valve stroke and valve open time can be reduced in a controlled manner.
A decisive factor in the operation of this conventional valve arrangement is the function of the solenoid valve, which must be open during the base circle phase of the cam in order to draw the hydraulic fluid from the accumulator into the working space which is then at its maximum volume. The solenoid valve must then close until the exact time when it must open once more to determine the closing time of the lift valve. An especially noticeable disadvantageous feature of this arrangement is that the control of the hydraulic fluid with the solenoid valve affects not only the intake of fluid but also the removal of fluid from the working space. Consequently, in the event of a malfunction of the solenoid valve, the characteristics of the valve control arrangement vary in an unpredictable manner. If the malfunction of the solenoid valve results in a permanent disconnection of the working space from the accumulator after the base circle phase of the cam, i.e., after the working space is completely filled, then the valve will operate continuously with its maximum stroke and maximum opening time. On the other hand, if the malfunction of the solenoid valve manifests itself in keeping the communication between the working space and the accumulator open, then the valve will be actuated with its minimum stroke and shortest opening time. In other words, despite the cost that a solenoid control valve involves, this conventional variable valve fails to provide a valve control arrangement having a defined default behavior.